The present invention relates to a roller hearth furnace, or kiln, for firing ceramic material.
The requirements to be met by the roller drive of such a furnace, and the design of a roller drive meeting such requirements, are described in an article by E. Wolf et al at pages 66-68 of the technical periodical entitled SPRECHSAAL, No. 1/1980. According to that article, to prevent bending or warping of the ceramic material, which is plastic in the high temperature range employed, as well as to broaden the range of formats, or shapes, to be handled by the furnace, the axial spacing between the rollers should be as small as possible.
But this requirement is, in a way, contrary to the requirement of keeping the free cross section between the rollers large so as to assure temperature equalization and good exposure of the material to be fired to the furnace gases. These mutually contradictory requirements result in the tendency of using rollers with as small a diameter as possible. This solution, however, has its limits because of the requirement for sufficient strength of the rollers at the high temperatures encountered.
The rollers employed are made of metal or ceramic materials and have diameters of approximately between 25 and 50 mm.
The above-cited article further presents reasons why persons skilled in the art do not drive a plurality of the rollers in such a furnace by means of a single rotating chain drive. The main reason for this is the requirement for a rotary movement which is as uniform and smooth as possible. Moreover, such a chain drive increases the difficulty of replacement of rollers; something that is frequently necessary in such furnaces and that must be feasible without interruption of furnace operation. Such replacement of rollers becomes necessary if rollers break, which occurs more frequently when rollers of ceramic material are used. Replacement of rollers may also become necessary, for example, because glazing material adheres to the roller surface, which leads to irregularities on the roller surface and to resulting reductions in the quality of the fired material.
The rollers of the individual longitudinal sections of the prior art roller hearth furnace are driven by a common shaft which extends in the longitudinal direction of the furnace. The synchronous transmission of torque from the common shaft to the individual rollers is effected by pairs of helical gear wheels, which have the know characteristic of operating smoothly because of their gradual engagement.
Such a drive for the rollers is structurally complicated. The encapsulation of such a drive requires a considerable amount of space. The rollers can be replaced only from the side of the roller hearth furnace opposite the drive shaft side if, with the customary encapsulated design of the drive assembly, the drive assembly box is not to be opened. When rollers break, the broken end on the driven side cannot be removed in a simple manner. Since the rollers are driven by means of helical gear wheels, axial thrusts are continuously transmitted from the drive shaft to the individual rollers which, in order to be absorbed in the associated bearings, requires increased expenditures for the bearings.